Mixers take two input signals and multiply them together to realize a frequency translation. Standard Gilbert Cell based mixers require some amount of DC (direct current) bias current for nominal operation. This bias current results in an undesired power dissipation and limits low-power performance. This problem is exacerbated when bipolar topologies are utilized and input bias currents are required which dissipate additional quiescent power. Moreover, the bias current cannot simply be scaled down in order to achieve low-power operation. Scaling the currents affects the required loading conditions, frequency response, and gain performance. In extremely low-power/low-current applications, the required passive loads prove unmanageably large and active loads at high frequencies are not currently practical in the art. Gilbert Cell mixers also have a limited output voltage swing based on biasing and load conditions.
The undesired input bias currents associated with bipolar topologies can be eliminated by utilizing CMOS topologies but the unwanted DC bias tail current still remains. Another alternative is to use CMOS switching mixer topologies which solve the DC bias issues but they have high levels of carrier feedthrough due to charge injection and some don't have the capability for rail-to-rail performance.
In receiver circuits, mixers translate a high input radio frequency (RF) to a lower intermediate frequency (IF). This process is known as down-conversion utilizing the difference term between the mixer's RF input and local oscillator input (LO) for low-side injection (LO frequency<RF frequency) or the difference term between the mixer's LO and RF for high-side injection. This downconversion process can be described by the following equation:fIF=±fRF±fLOwhere fIF is the intermediate frequency at the mixer's output port, fRF is any RF signal applied to the mixer's RF input port, and fLO is the local oscillator signal applied to the mixer's LO input port.
Ideally, the mixer output signal amplitude and phase are linearly related to the input signal's amplitude and phase and independent of the LO signal amplitude. (Note this is in contrast to a multiplier where the output amplitude changes with LO amplitude.) Using this assumption, the amplitude response of the mixer is linear for the RF input and is independent of the LO input amplitude.
However, mixer nonlinearities produce undesired mixing products called spurious responses, which cause undesired signals reaching the mixer's RF input port to produce a response at the IF frequency. The signals reaching the RF input port do not necessarily have to fall into the desired RF band to be troublesome. Many of these signals are sufficiently high in power level that the RF filters preceding the mixer don't provide enough selectivity to keep them from causing interference. When they interfere with the desired IF frequency, the mixing mechanism can be described by:fIF=±mfRF±nfLONote that m and n are integer harmonics of both the RF and LO frequencies that mix to create numerous combinations of spurious products. In reality, the amplitude of these spurious components decreases as the value of m or n increases.
Knowing the desired RF frequency range, frequency planning is used to carefully select the IF and resulting LO frequency selections to avoid spurious mixing products whenever possible. Filters are used to reject out-of-band RF signals that might cause in-band IF responses. IF filter response following the mixer is specified to pass only the desired frequencies thereby filtering the spurious response signals ahead of the final detector. Spurious responses that appear within the IF band will not be attenuated by the IF filter.
Many types of balanced mixers reject certain spurious responses where m or n is even. Ideal double balanced mixers reject all responses where m or n (or both) is even. The IF, RF, and LO ports are mutually isolated in all double balanced mixers. Therefore, with properly designed baluns, these mixers can have overlapping RF, IF, and LO bands.
In fundamental terms, mixer design involves trade-offs among several performance objectives and specifications, including balanced operation, noise figure, second and third order intermodulation (IM) distortion, power drain, and cost. Although a mixer described in U.S. Pat. No. 6,603,964 achieves many of these performance objectives, no existing mixer design achieves an exceptional level of performance for each of the objectives simultaneously.